1. Field of the Invention
The present invention relates to improvements in protective coatings for electroerosion print media.
2. Description of the Prior Art
Electroerosion printing is a well known technique for producing markings such as letters, numbers, symbols, patterns, such as circuit patterns, or other legible or coated indicia, on a recording material. This is typically performed by computer-output controlled printing which involves removal of an aluminum layer from selected areas of the recording material as the result of spark initiation.
The surface which is locally eroded or removed during writing to provide such indicia on the recording material is a thin film of conductive material, usually a vacuum deposited aluminum layer, which vaporizes in response to the high localized temperature associated with sparking (arcing) initiated by applying an electric current to a stylus, more generally to multiple styli, typically tungsten wire in a printing head, having electrical contact between the surface of the thin conductive film and the tip of the stylus.
Electroerosion printing is effected when the stylus, styli or print head sweep across the surface of the recording medium. Electrical writing signals are fed to the stylus, styli or print head to provide controlled electrical pulses which generate sparks accompanied by high local temperatures at the surface of the recording material causing a selective disruption and evaporation of the aluminum film in areas of the recording material. The locations from which material has been removed correspond to the indicia or images which are to be recorded.
Electroerosion recording materials and printing processes are useful to directly produce human readable images, photomasks, etc. Special paper and polymer substrates have been employed which are coated with a black organic underlayer and then the electroerodable conductive layer, typically a metal film such as vapor deposited aluminum film, is deposited. For details on such materials heretofore used in electroerosion printing, see U.S. Pat. No. 4,082,902, A. Suzuki et al, U.S. Pat. No. 3,786,518, B. Atherton, and U.S. Pat. No. 4,086,853, M. Figov et al.
The use of electroerosion techniques for high resolution output that can be employed as a "direct negative" as well as a "direct master" or a camera-ready copy for offset lithographic printing requires fabrication of special recording media for problem-free and scratch-free printing. When the usual transparent polyester substrate with a vacuum deposited thin aluminum film is employed as a recording medium for high resolution electroerosion printing, severe mechanical scratching of the aluminum layer is observed which may be due to plastic deformation of the substrate during printing and/or due to slight differences in the pressue on individual styli in a multi-styli head. The aluminum film apparently cannot withstand the high strains generated when the support or substrate is deformed and therefore suffers mechanical failure resulting in scratching. Also, there are occasions when the styli cold-weld to the thin aluminum conductive layer so that the structure suffers shear either at the aluminum-substrate interface or below it in the substrate itself.
It has been recognized for some time, therefore, that the use of a lubricant and/or protective overcoat on the surface of an electroerosion recording material would be helpful to reduce this problem of scratching.
The prior art has utilized lubricants comprising long chain fatty acids. However, even with the use of such lubricants, some electrode scratching of the removable layer of the electroerosion recording material still remains. Accordingly, efforts continue to be directed to finding a superior lubricant and/or protective layer composition for the surface of electroerosion recording materials.
U.S. Application Ser. No. 454,743, Cohen et al, filed Dec. 30, 1982 and now abandoned describes electroerosion recording media suitable for high resolution printing to produce high quality "direct negatives" and short run "offset masters".
Several additional prior art patents relevant to electroerosion recording or printing are discussed below.
U.S. Pat. No. 2,983,220, Dalton et al, discloses a lithographic coating on an electroerosion recording sheet. The coating may be a copolymer binder system containing zinc oxide and zinc sulfide.
A layer containing a conductive material, such as graphite, is disclosed in U.S. Pat. No. 3,048,515, Dalton et al.
An electroresponsive recording blank having a removable masking layer containing a luminescent material is disclosed in U.S. Pat. No. 2,554,017, Dalton et al.
Other prior art providing further general background in the field of electroerosion printing includes U.S. Pat. No. 3,138,547, Clark, and U.S. Pat. No. 3,411,948, Reis.
High temperature lubricants comprising graphite in oil are also known as disclosed in U.S. Pat. No. 3,242,075, Hunter.
U.S. Pat. No. 3,514,325, Davis et al, discloses an electroerosion recording material in which a surface layer of crosslinked binder containing conductive particles such as zinc oxide is placed on top of a thin aluminum layer.
U.S. Pat. No. 4,305,082, Kusakawa et al, describes an electroerosion recording paper in which a resistance layer may be provided over a thin conductive aluminum film.
U.S. Pat. No. 4,304,806, Anderson et al, describes information-carrying disks coated with an abrasion resistant polymer layer derived from an epoxy-terminated silane.
U.S. Pat. No. 3,002,854, Bril et al, relates to a process involving treatment of a solid surface with titanium organic compositions to improve adhesion with other surfaces.
U.S. Pat. No. 3,082,117, Schilly, relates to coated films, particularly to organic based films such as polyethylene terephthalate, having a continuous coating of a heat-sealable thermoplastic organic polymeric coating which involves applying a continuous coating of a solution of a titanium organic compound from the group consisting of hydrolyzable organic titanium esters and reaction products of acetylacetone with an alkyl titanate.
U.S. Pat. No. 3,951,882, A. H. Markhart et al, relates to dielectric coating compositions containing an organophilic clay pigment prepared by treatment of clay with an organotitanium compound.